In modern times, deep ocean circulation has been governed by roughly equal fluxes of deep water formed in the North Atlantic and in the Southern Ocean around Antarctica. However, in the warmer climate of the early Cenozoic era, ~55 million years ago (Ma), the Southern Ocean was the principal deep-water source, and a very different mode of global heat transport would have been operative. Thus, the precise timing of the transition from unipolar to bipolar deep-water circulation is of interest in climate as well as ocean studies.
Via and Thomas have analyzed neodymium isotopic abundance data acquired from Southern Hemisphere sites that they chose to discriminate clearly between the two circulation patterns. They find that deep-water production in the North Atlantic began ~33 Ma, as indicated by the divergence of isotope records from the more southerly Maud Rise and the more northerly Walvis Ridge. Among other implications, this finding suggests that the Greenland-Iceland-Faeroes Ridge--the sill over which deep water from the North Atlantic must flow in order to reach the southern portion of the ocean--must have deepened at that time. The dating of this event should improve understanding of climate changes that occurred during this dynamic interval of global cooling and tectonic evolution of the Atlantic basin. -- HJS
Geology 34, 441 (2006).
